Printed circuit boards are manufactured by plating a thin layer of copper on an epoxy-glass cloth laminate board of uniform thickness. A predetermined pattern of holes is then drilled to accommodate various electrical components. A film of partially polymerized photoresist plastic is deposited on the laminate over the copper plating. Photoresist films are commonly composed of acrylic resins, polyvinyl cinnamates, diazo compounds, phenol-formaldehydes, or other similar film-forming materials. This film is further polymerized, or cross-linked, by the action of ultraviolet light, into a hard chemically resistant film.
By masking with an appropriate glass or plastic material the resist film is selectively hardened by the exposure to light in specific predetermined areas, while the resist film in the masked areas is left unchanged. The unchanged resist film is then dissolved in a "developer" by a solvent such as 1,1,1-trichloroethane or a solution of butoxyethanol and sodium carbonate or similar solutions. The copper in the cleaned areas may then be removed by etching or additional copper and other metals may be plated thereon. In either event, it then becomes necessary to remove the exposed hardened resist film from the laminate.
It is known that such resist films can be removed by the action of strong organic solvents, such as methylene chloride or trichloroethylene.
It is known also that many paint and varnish removers are based on methylene chloride. Many of these also contain an alcohol and one or more other additives. Thus, U.S. Pat. No. 3,650,969 discloses a composition for removing paint which comprises dichloroalkanes of 1 to 2 carbon atoms, an aliphatic monohydric alcohol containing 1 to 3 carbon atoms, and hydrogen flouride and water. U.S. Pat. No. 3,600,322 discloses a paint remover composition containing methylene chloride, methanol and a quaternary nitrogen cellulose ether.
Other patents employing methylene chloride with alkanols and various other additives are U.S. Pat. Nos. 3,147,224; 3,075,923; 4,269,724 and 4,278,557.
A method of removing resist from printed circuit boards is described in U.S. Pat. No. 3,789,007 wherein the board is treated with a mixture of 85 to 97% by weight of methylene chloride with the balance being methanol.
Other methylene chloride-containing photoresist stripper compositions are disclosed in U.S. Pat. No. 3,625,763; 3,813,309 and 3,625,763.